The present invention relates to an apparatus for shaping monocrystal ingots into right circular cylindrical form with high precision. More particularly, the invention relates to an apparatus which comprises an automatic center axis determining unit, an automatic external cylindrical grinding unit, and an automatic diameter control unit.
A semiconductor monocrystal ingot which is obtained in a crystal pulling method such as the Czochralski method and the floating zone method has a general shape of circular cylinder with cone tail ends. The cone tail ends of the monocrystal ingot are severed from the main body of the monocrystal ingot in order that it can be held firmly at the end furing a subsequent cylindrical grinding process. After the cylindrical grinding, the cylindrical monocrystal ingot is sliced into thin plates called "wafers" in a manner such that the slicing direction is normal to the center line of the ingot. However, a monocrystal ingot has irregular or uneven surfaces such that when it were cut across along a plane including the center line, the cross section would have a waved contours across its length. Therefore, the wafers sliced from the ingot would naturally have varying diameters. This is disadvantageous from the view point of the product quality control. In order to obtain wafers of the same diameter, the ingot must be ground by an external cylindrical grinding machine into the shape of a right circular cylinder, prior to the slicing operation.
To obtain a right circular cylinder with the largest possible diameter from an irregularly contoured cylindrical ingot, it is the primary importance to determine the optimal center axis, based on which the grinding operation is to be conducted. More specifically, the ingot is to be rotated about the optimally determined center axis while it is externally ground. After determining an optimal center axis in an irregularly contoured circular cylinder ingot, two points are marked through which the determined center axis passes, one point in a circular end face of the cylinder and the other point in the other circular end face of the cylinder. To attain the best yield of the right cylindrical ingot, the optimum center axis should be determined such that the uniform diameter of the wafers sliced from the cylindrical monocrystal ingot will be the largest possible. To attain this end, methods were proposed which include Japanese provisional patent publication (kokai) No. 57-66909 (1982), Japanese laid open publication (kokoku) No. 60-17682 (1985), and Japanese laid open publication (kokoku) No. 61-33446 (1986).
According to Japanese provisional patent publication (kokai) No. 57-66909, the center axis of a log is determined by first detecting the contours of at least three cross sections taken normally to the provisional center axis at arbitrarily selected locations including the two end faces; finding the radii and the centers of the maximum inscribed circles in the respective cross sections; then obtaining the maximum right circular cylinder confined by the at least three maximum inscribed circles; and finally finding the center axis of the confined maximum right circular cylinder.
According to Japanese laid open publication (kokoku) No. 60-17682, the center axis of a log is determined in the following manner: a log is rotated about a provisional center axis; light beams are applied to the log normally to the axis thereof from a side position; that light beam which is exactly tangent to the surface of the log is detected every time the log is rotated at intervals of a predetermined angle at a plurality of locations taken along the length of the log; a polygon is obtained from the detected tangent beams at each detection location; the maximum right circular cylinder confined by the polygons is obtained; and the single center line of the confined maximum right circular cylinder is obtained, which is then taken as the center axis about which the log is rotated and externally ground.
According to Japanese laid open publication (kokoku) No. 61-33446, the center axis of a log is determined in a fundamentally similar manner as disclosed by Japanese provisional patent publication (kokai) No. 57-66909 above.
However, the fact is that no equipment has been proposed with which it is possible to effectively practice these methods in obtaining an optimal center axis of a generally cylindrical body. Besides, in the field of semiconductor manufacturing, there has been made no proposal of an apparatus with which it is possible to practice a method for determining the optimum center axis of a monocrystal ingot which is to be ground into a right circular cylinder.
Further, after the optimal center axis is determined in the monocrystal ingot of a generally cylindrical shape, the irregularly waved surface of the ingot is ground in a manner that the ingot will have right circular cross section and straight lengthwise contour, so that a right circular cylinder having a predetermined diameter is obtained, by means of an external cylindrical grinding machine. The thus obtained cylindrical monocrystal ingot is sliced in a direction such that each cut face is approximately normal to the center axis of the ingot, that is, in a crystallographically determined direction. Thereby, thin circular plates are obtained, which are wafers, and lapping and etching operations are applied to the both sides of each wafer. Thereafter, one side of each wafer is polished until specular gloss is obtained.
Conventionally, in the grinding operation of the ingot by means of the external cylindrical grinding machine, the measurement of the diameter, based on which the control of the grind amount is conducted, and the setting of the grind amount were performed in an open-loop control manner, which means that once the grinding amount is set at the onset of grinding it is not modified responsive to the result of grinding. Although a numerical control (NC) system may be employed for setting of the grinder, the measurement of the diameters and others was performed by a man.
In the conventional open-loop control manner, an error in setting the grinder in grind position, the wear of the grinder (e.g., grinding stone), and other factors contribute to a failure in obtaining the aimed diameter with precision. Also, since the setting of the grind amount and the measurement of the ingot diameter are all performed manually by human operators, the precision must depend on the craftsmanship of the operators, which is not promising to obtain constant precision, and a considerable amount of expensive human labor is required.